In general, in a digital camera, exposure of an object is controlled by setting the opening ratio of an aperture that is provided at a lens, the electronic shuttering amount of a CCD, which is an imaging device, and the gain of an imaging signal output from the CCD. Many digital cameras use a hill-climbing auto-focusing device as an auto-focusing (hereinafter abbreviated as “AF”) device.
Here, auto-focusing means to automatically bring a taking lens into focus with an object. In auto-focusing, the distance to the object is measured to obtain focusing. Hill-climbing auto-focusing provides the principle of measuring distance in auto-focusing in which the location of a lens where a high-frequency component having a luminance level detected by the CCD becomes a maximum is the location of light focus.
Therefore, when an object in a dark background does not have sufficient contrast, the position of the object can no longer be determined, so that the location of light focus cannot be adjusted. To overcome this problem, when there is not sufficient contrast because the luminance level of the object in a dark background is low, auto-focusing is carried out. During the auto-focusing, an exposure controlling operation which increases the gain of an imaging signal is carried out. However, when the luminance level is very low because the object is at a place where there is very little light around it, such as in the dark, the luminance level remains insufficient even if the exposure controlling operation which increases the gain of an imaging signal is carried out, so that it may not be possible to increase the precision of the auto-focusing. To overcome this problem, in recent digital cameras, an object is illuminated with AF auxiliary light in order to provide light when the luminance level is insufficient. In such digital cameras, an auxiliary light emitter for emitting AF auxiliary light is built in the camera body, or is built in a flash unit for illuminating an object to properly expose it when it is photographed in a dark environment or against the sun and is mounted to the camera body when necessary.
Here, the amount of exposure control (opening ratio of an aperture, electronic shuttering amount of a CCD, gain of an imaging signal) is determined by comparing the luminance level of the object with a target value that is stored in RAM, which is a memory that needs to be sequentially subjected to a writing operation, and feeding back the result of the comparison. Therefore, when the luminance level of the object is not a proper level, the amount of exposure control also becomes an improper amount.
In related digital cameras, although the amount of exposure control is updated based on the luminance level while the object is being monitored before photographing it, the amount of exposure control is not updated during auto-focusing. In the case where auxiliary light is emitted, that is, the background is bright, when the luminance level of the object at the time of monitoring and that at the time of auto-focusing are the same, no problems arise even if the amount of exposure control is not updated. However, in the case where the luminance level of the object at the time of monitoring and that at the time of auto-focusing are not the same, when the luminance level of the object at the time of monitoring is used, the exposure is no longer controlled by a proper value. For this reason, in related digital cameras using AF auxiliary light, the precision of auto-focusing cannot be increased. In order to mitigate this problem and perform a proper exposure controlling operation even after emission of auxiliary light, it is necessary to compute the amount of exposure control again by detecting again the luminance level of the object after the emission of auxiliary light.
As mentioned above, when photographing an object with an AF camera using a hill-climbing method, in order to focus an image when it is being photographed, the object needs to have sufficient contrast. There are many situations where the object does not have sufficient contrast, such as when the outline of the object is not clear. When the object cannot have sufficient contrast because its luminance level is low, the image can be focused by increasing the luminance level. In other words, during auto-focusing, an exposure controlling operation which increases the gain of an imaging signal compared to that during monitoring is carried out. When sufficient contrast still cannot be provided due to insufficient luminance level even after this exposure controlling operation has been carried out, the object is illuminated with AF auxiliary light of, for example, an LED, to increase the luminance level of the object for achieving focus. In a camera including an auxiliary light emitter for emitting AF auxiliary light, when the amount of light of the place where the object exists is determined as being low during focusing, a controlling operation is carried out so that auxiliary light is emitted.
In general, the operation modes to the time of recording an image in a digital camera changes from a monitoring mode to a scan AF mode to a photographic mode in that order. Here, the monitoring mode is a mode in which data of an image is displayed on a display section without storing it on a storage medium (in DRAM) to monitor an object.
The scan AF mode is a mode for achieving focus before recording an image. For example, in the scan AF mode, a shutter button is pressed halfway. Here, in order to make it easier to perform AF, the aperture is brought close to an open state and the focal depth is made small. When the luminance level of an object is low, an exposure controlling operation which increases the gain of an imaging signal is carried out to obtain sufficient contrast. The photographic mode is a mode for recording an image.
Conventionally, during monitoring mode, an exposure controlling operation (opening ratio of an aperture, electronic shuttering amount of a CCD, and gain of an imaging signal) is carried out for each frame rate. However, when the operation mode of the camera changes from the monitoring mode to the scan AF mode, an exposure controlling operation is not carried out again in the scan AF mode. Therefore, when auxiliary light is emitted, the amount of exposure control does not change even though the luminance level changes, so that the exposure when auxiliary light is emitted is no longer carried out by a proper amount. This results in a problem in that the precision of the auto-focusing is reduced.
The present invention has been achieved to overcome problems such as those mentioned above, and has as its object the provision of a method for controlling exposure of a digital camera which, by controlling an exposure operation again even after emission of auxiliary light for changing luminance, makes it possible to perform proper exposure even when auxiliary light is emitted, so that the precision of auto-focusing is increased.